California is facing what could be the state's biggest outbreak of pertussis since 1958, its top public health official said.

California is facing what could be the state's biggest outbreak of pertussis since 1958, its top public health official said.

"Whooping cough is now an epidemic in California," said Mark Horton, MD, MSPH, director of the California Department of Public Health, in a statement. "Children should be vaccinated against the disease and parents, family members, and caregivers of infants need a booster shot."

As of June 15, 910 cases had been confirmed in the state. Another 600 suspected cases are currently being investigated by local health officials, the statement indicated.

The number of pertussis cases is now on a pace to surpass the total of 3,182 seen in the most recent major outbreak, which occurred in 2005, said Ken August, spokesman for the department.

August told MedPage Today that 1,200 cases were recorded in 2005 as of mid-June, meaning the state would probably exceed that this year.

The highest yearly total on record is 3,837 cases, seen in 1958.

So far this year, five children have died, all infants younger than three months.

Blaise Congeni, MD, of Akron Children's Hospital in Ohio, said he wasn't surprised, as higher-than-usual numbers of pertussis cases have been reported across the country.

"We are having an outbreak in Ohio," though not as severe as in California, he said in a phone interview.

Oregon public health officials have also reported an unusually high number of pertussis cases in that state.

Both Congeni and August noted that pertussis tends to wax and wane in cycles. But gaps in vaccination coverage may also be playing a role, particularly in California.

"California is the epicenter of vaccine refusal" in the U.S., said Congeni.

Although outright refusal as well as so-called designer schedules for vaccinations that deviate from evidence-based recommendations are occurring throughout the country, including Ohio, "it's not as much as what my colleagues in California tell me," Congeni said.

August explained that California requires that children receive the full slate of vaccinations for pertussis, measles, and other infectious diseases before they can attend school. But the requirement is waived if parents file a "personal belief exemption" (PBE), which need not be based on religion or medical necessity.

He said that the overall rate for PBEs among the state's roughly 7,200 schools is about 2%.

But rates are much higher in some schools. Records for 2009 indicated that close to 175 schools had PBE rates of 20% or more. A few had rates above 70%.

Researchers have found that vaccination rates of at least 93% are needed to ensure so-called herd immunity against pertussis, which prevents the disease from spreading quickly to unvaccinated individuals.

However, August said that some parents may use the PBE to withhold just one vaccine from their children. Others may change their minds after filing a PBE and have their children fully vaccinated.

He also noted that, in severe outbreaks of vaccine-preventable diseases, local school officials can declare a health emergency and require that all schoolchildren be vaccinated. Parents who insist on withholding vaccination must then keep their children out of school.

August said he was unaware of any such declarations in the face of the current pertussis outbreak.

The California public health department first warned of a pertussis spike in April, when the case count was running double the rate seen at the same time in 2009.

The current outbreak in California still pales against a major epidemic that swept England and Wales in the late 1970s and early 1980s, after pertussis vaccination coverage dropped to 30% in the wake of controversy over alleged vaccine reactions.

From January to September 1982, for example, nearly 50,000 children and adults contracted the disease. As many cases were reported in a single week as are expected in California for the entire year in 2010.

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